Caustic treating process



Oct. 15, 1963 Filed Feb. 12, 1959 2 Sheets-Sheet 1 l5 TREATED] ALKALINE-S3Efi$zfi I 25 24 awe I VENT EXTRACTIQN 2o DISULFIDES TOWER I OXIDATION.U UNIT 1 HYDROCARBON II ALKALINE 5, L l2 RECOVERY 22 TOWER WASH oIL 24REGENERATED SPENT ALKALINE SOLUTION ALKALINEPHENOLIC SOLUTION FIGURE-lCharles 0. Cole Jackson Eng Georges G. Lukk Inventors Charles B. RupurNorman L. ShIpley By Rims. QMJMLL Agenr Oct'. 15, 1963 c. o. COLE ETAL3,107,213

CAUSTIC TREATING PROCESS Filed Feb. 12, 1959 2 Sheets-Sheet 2 ousnvo NIs'llo GIOV maoaad "loA Inventors Agent VHlHdVN GBLVHHL Charles 0. ColeJackson Eng Georges G. Lukk Charles B. Rupur Norman L. ShIpIey NISNVldVOHHW HO S'IONEIHdOIHJ.SOI'ION3Hd :IO NOLLOHOHH .LNBOHBd UnitedStates Patent 3,107,213 CAUSTIC TREATING PROCESS Charles 0. Cole,Jackson Eng, Charles B. Rupar, and Norman L. Shipley, Sarnia, Ontario,Canada, and Georges G. Lukk, Westfield, N.J., assignors to Esso Researchand Engineering Company, a corporation of Delaware Filed Feb. 12, 1959,Ser. No. 792,895 3 Claims. (Cl. 208-230) The present invention dealswith the caustic treating of petroleum fractions. More particularly, itconcerns a method for removal of mercaptans and thiophenols in a mannermaking maximum use of caustic while reducing problems of waste causticdisposal.

The treatment of various hydrocarbon streams with alkaline solutions soas to reduce their mercaptan and thiophenol contents is well known inthe art. Mercaptans are undesired due to their noxious smell.Sulfur'containing compounds are in general undesirable since thepresence of sulfur adversely affects stability and combustioncharacteristics of fuels. In addition to removing mercaptans andthiophenols, the alkali extraction will also remove phenolic compoundssuch as phenol, the cresols, and higher phenolics, normally present inthe petroleum fraction to be treated. In treating a typical crackednaphtha, for example, these phenolics will comprise up to 1 wt. percentof the hydrocarbon oil While the total quantity of mercaptans andthiophenols may be about 0.1 wt. percent or less. In the conventionalmethod of caustic washing such a naphtha fraction, up to 90 wt. percentor more (based on phenolics present in the 'naphtha) will be extractedalong with mercaptans and thiophenols.

The extraction of phenolics presents several significant problems. Ithas been ditlicult'to dispose of the spent caustic containing phenolswhich continuously pass into the. alkali solution during extraction.Lange amounts of a high phenolic content, caustic stream cannot betolerated in rivers, streams, etc., due to their harmful effect onaquatic life. Thus, heretofore it was required to treat such caustic inorder that their phenolic content be considerably reduced. Additionally,the extraction of phenols is not of particular value and the alkaliconsumed thereby is essentially wasted with regard to its availabilityfor the removal of mercaptans and thiophenols.

In accordance with the present invention, means are taught wherebyoptimum use is made of available alkali, and minimum difiioulties indisposal of alkaline solution are incurred. More particularly, asufficient quantity of phenolics is added to the alkaline solution to beused for extraction of hydrocarbon oil so that the concentration ofphenolics in solution is in equilibrium with the phenolic compoundspresent in the hydrocarbon oil to be treated. The free alkali content ofthe alkaline solution is maintained at a very low value, i.e. less than2 Wt. percent. When the alkaline solution containing an equilibriumconcentration of phenolics is contacted with the hydrocarbon oil,thiophenols and mercaptans are extracted Whereas the solution does notextract phenolics from the oil. Since thiophenols and mercaptans aremore acid than phenolics, the equilibrium relationships are such thatthe available small quantity of free alkali will essentially only beutilized for removal of undesired sulfur-containing compounds as opposedto the extraction of phenolics.

In a preferred embodiment, spent alkaline solution, i.e. solution richin extracted mercaptans and thiophenols, is

3 1 ,2 13 Patented Oct. 15, 1963 "ice regenerated by oxidizing themercaptans and thiophenols to disulfides, separating out the disulfide,and recycling alkaline solution containing equilibrium quantities ofphenolics to the hydrocarbon extraction zone.

The present invention is generally applicable to the alkaline treatmentof oils containing mercaptans and phenolics. Among fractions suitablefor treatment are catalytically cracked naphtha, heavy virgin naphtha,kerosene, and the various middle distillates.

The benefits of the present process are as follows:

(l) Maximum use of alkali to remove undesirable compounds such asmercaptans and thiophenols.

(2) Increased product yields since phenolics remain in the hydrocarbonoil treated.

(3) Spent caustic disposal problems are markedly reduced since thequantity of phenolic-containing, spent alkaline solution ultimately tobe withdrawn from the system is considerably less than in conventionalprocesses.

(4) Data has shown that the presence of phenolics in gasoline, etc. arenot detrimental. In fact, certain phenolics are natural inhibitors.Thus, the oil treated in the present process would normally require lessantioxidant inhibitor addition since natural inhibitors have not beenremoved.

It is to be clearly noted that the present invention is clearlydistinguishable from the conventional use of solutizers, as for exampleshown in U.S. Patent 2,351,- 467. Although it has previously beensuggested to add limited quantities of various materials to alkalinewashes, e.g. amines, phenols, etc., the preservation of an equilibriumconcentration of phenolics in said alkaline solution so as to preventsubstantially any extraction of phenolics from the hydrocarbon to betreated was not heretofore contemplated. In the prior art systems, thephenolic concentration in said wash solution was substantially belowequilibrium requirements and/ or the relatively large quantity of freealkali employed resulted in substantial phenol extraction from thehydrocarbon oil treated. The greater the amount of free alkali, thegreater will be the degree of phenolics extraction from the hydrocarbonoil.

By way of nomenclature, the term alkaline solution denotes the basic (asopposed to acidic) solution formed by dissolving alkali-metal compoundsin water. Though hydroxides of sodium and potassium are preferred,various salts such as carbonates may also be employed. The term freealkali is used to denote the quantity of unreacted alkali present insolution, i.e. the quality of alkali available for interaction withmercaptans, thiophenols and phenolics.

The various aspects and modifications of the present invention will bemade more clearly apparent by reference to the following description,examples and accompanying drawings.

FIGURE 1 depicts a preferred mode of operation whereby extractedmercaptans are removed while regenerating alkaline solution for furtherextraction.

FIGURE 2 illustrates experimental results derived from the study ofalkaline extraction of a hydrocarbon oil containing mercaptans,thiophenols and phenolics.

Turning to FIGURE 1, the system shown therein basically comprisesextraction tower 10, oxidation or regeneration vessel l1, and akalinerecovery tower 12.

It is desired to extract a light, catalytically cracked naphtha, boilingin the range of to 330 F. The hydrocarbon contains about 0.02 wt.percent thiophenols, 0.2 wt. percent phenolics and has a mercaptancontent of 0.01 wt. percent. It is desired to reduce the thiophe- 1101and mercaptan contents to less than about 0.002 wt. percent and 0.002wt. percent respectively.

in the caustic is sufficiently great so as to be in equilibriurn' withthe phenolics, such as phenol and cresols, present in the hydrocarbonoil to be treated. Normally about volumes of hydrocarbons are treatedper volume of caustic introduced by line 14, but each volume of freshcausticpheno-lics solution can be used to treat up to 1000 volumes ofoil before regeneration in unit 11, as will be later described. 'I'healkaline solution of line 14 can be derived in large part from recyclecaustic withdrawn by lines 16 and :17.

Tower 10 can be any conventional extraction apparatus. It preferably isa multi-contactingstaged unit, the staging and nature of the causticsolution introduced thereto being such thatthe treated oil removedthrough line contains at most the concentration of mercaptans andthiophenols indicated previously. The Washed oil will containsubstantially the same quantity of phenolics, is. 0.2 wt. percent, asthe initial oil feed.

Alkaline solution containing extracted mercaptans and thiophenols iswithdrawn by line 16. About 90% of the extract can be recycled by line17 for further contact with the oil to be washed. Continuously, orperiodically if so desired, mercaptans and thiophenols must be removedfrom the caustic treat solution in order to 'allow for furtherextraction of sulfur-containing compounds. This is preferably done bymeans of oxidation unit 11, although in the broadest aspects of thepresent invention, any regeneration process efifecting such a result isapplicable.

The spent caustic is thus sent to unit 11 wherein air or otheroxygen-containing gas introduced through line 19 serves to convert themetallic salts of the extracted mereaptans and thiophenols to theirdisulfide forms, thus freeing sodium hydroxide for further extraction,i.e. for further interaction with the sulfur compounds present in thehydrocarbon oil. Air regeneration of caustic, per se, is Well known inthe art. Very briefly described, unit 11 operates at a temperature of 50to 200 F. e.g. 100 F., approximately 5 to 10rnoles of oxygen beingemployed for each mole of mercaptans and thiophenols. Though not shown,for the sake of simplicity a number of contacting passes between thecaustic and the air are generally employed. Gases are vented throughline 20.

Though mercaptides are converted by oxidation, there is no substantialeifect on the concentration of phenols (and carboxylic acids) present inthe alkaline solution. Thus thealkaline solution withdrawn by line 21will comprise substantially the same concentration of phenolics as thesolution in line 18, as well as containing disulfides. It has a freecaustic content of about 0.01 to 1 wt. percent. Since it is desired tohave, a low concentration of free caustic in the extraction tower, arelatively simple regeneration step is readily employe Disulfides areremoved from the alkaline solution in tower 12. A wash oil substantiallyfree of thiophenols and mercaptans is introduced into tower '12 by inlet22, thereafter countercurrently contacting the regenerated alkalisolution and extracting disulfides therefrom. The wash oil is normally avirgin or cracked naphtha fraction and is preferably a stock which is tobe hydrofined subsequently. Tower '12 operates at ambient temperature,about 0.1 to 10 volumes of wash oil being employed per volume ofregenerated alkaline solution. The spent wash oil containing disulfidesis Withdrawn by line 23 and may be sent to hydrofining, where thedisulfides are split into hydrocarbons and hydrogen sulfide.

. The regenerated alkaline solution is thereafter recycled to extractiontower 10 by line 24.

Though not normally necessary or desired (from the 4 standpoint ofeasily maintaining phenolics equilibrium), a very small quantity offresh alkali may be added to the system, thus supplementing theregeneration step. Since ultimately the concentration of carboxylicacids will tend to limit the life of the alkaline solution, the freshcaustic addition preferably will operate in conjunction with the purgingof spent, high acid oil content solution, periodically done through line25. Such a high acid all solution, which represents the alkalinesolution to be disposed of, is readily saleable for the recovery of theacid oils While dilute solutions of acid oils are not. Thus, none orvery little solution ultimately need be discharged into streams or thelike.

Various modifications may be made to the system described above.Generally, when starting up the process, an equilibrium concentration ofphenolics in the caustic will be secured by washing a minor portion ofthe overall oil feed with caustic until the phenolics build up to anequilibrium value. The resulting alkaline is then used for the practiceof the present invention. Of course, extraneous phenolics may be addedto the caustic if desired. Though it is theoretically desired to preventany extraction of phenolics from the oil during the alkaline wash, it isrealized that practical inefiiciencies may result in some change ofphenolics concentration. The term substantially constant as applied tophenolics concentration denotes this small degree of inherent departurefrom desired results.

Table 1 presents a compilation of data applicable to the systemheretofore described.

TABLE 1 Preferred range Broad range 5 to 1,000.--. 5 to 20. 50 to 200...to 120.

0.01 to 2.0--- 0.02 to 1.0.

0.25 170 10-.-. 5 to 10. 50 to 80 to 120.

Various experiments illustrating the practicality of the presentinvention will now be described. With specific reference to FIGURE 2, anexperiment was conducted with respect to illustrating that there existsa range of conditions within which an alkaline solution will effectivelyremove thiophenols and mercaptans from a hydrocarbon without extractingphenolic materials there from.

A catalytically cracked naphtha containing 0.134 wt. percent phenolics,0.011 wt. percent thiophenols and having a mercaptan no. of 5.4(equivalent to a 0.007 wt. percent mercaptan sulfur'concentratiou) wastreated with an 18 B. solution of caustic. The caustic had previouslybeen used in plant operation to contact about 1000 volumes of naphthabefore being employed in the present experiment. Thus, it contained 31vol. percent acid oils and had approximately 2% free caustic soda.

The acid oil test (neutralizing of caustic'with'sulfuric acid) was adirect measure of the'phenolic content since the naphtha did not containcarboxylic acids.

The naphtha and caustic solution were then contacted The degree ofmercaptan and thiophenol reduction, as

well as the degree of phenols extraction and buildup of acid oilconcentration, is depicted in FIGURE 2.

With the initial treats, the caustic reduced the micphenol content ofthe naphtha by 100 percent and the mercaptan content by 80, whileextracting 40% of the phenolics in the naphtha. With additional treats,the thiophenols and mercaptans were preferentially extracted. The degreeof extraction of phenols continuously decreased (with a consequentbuildup of acid oils in the caustic), until when each volume of initialcaustic had treated 200 volumes of naphtha, equilibrium was reachedbetween the phenolics in the caustic and in the naphtha, and thus nofurther phenolics were extracted. However, it is seen that during theinterval of 200 to 250 volumes of naphtha/volume of caustic, thepercentage of thiophenol and mercaptan reduction remained fairlyconstant. Ultimately, the caustic became saturated with these materialsand their degree of removal decreased with continued use of the caustic.

It is therefore evident that there exists an interval (indicated as thecross-hatched area of FIGURE 2) wherein caustic did not extractphenolics from the oil but yet satisfactorily removes thiophenols andmercaptans. Thus by operating in this interval, maximum utilization offree caustic is possible. In the present example, about 0.4 wt. percentfree caustic was present during this range. By controlling the additionof regenerated caustic, these treating conditions, i.e. 40 volumepercent phenolics in caustic and about 0.4 wt. percent free caustic, maybe maintained and caustic will be expended for extraction of mercaptansand thiophenols only.

It is to be noted that the use of a conventional phenol solutizer wouldresult in conditions corresponding to those of less than 200 volumes ofnaphtha per volume of caustic as shown in FIGURE 2.

Table 2 illustrates the oxidative regeneration of a caustic solutionemployed to treat a light catalytic naphtha feed oil.

TABLE 2 Air Regeneration of Spent Caustic From Treating of Light Cat.Naphthzz Mercaptan Number Reduction Conditions:

Charge 1500 ml. Air rate 0.15 s.c.f.m. Temperature 150 F. AgitationMechanical agitation and air blowing. Sampling Hourly check onmercaptanno.

Regeneration Data:

1 Caustic sample was acidified in presence of n-heptane of zeromercuptan no. Aliquot of n-heptane was titrated potentiometrically formercaptan content. Bracketed figures were obtained by directpoteutiornetric titration of Caustic.

As seen above, the mercaptan no. of the caustic was markedly decreased,thus freeing caustic which had been chemically held by the mercaptides.The free caustic content was increased approximately 2.6 fold. The acidoil content of the caustic was not materially affected by the oxidativeregeneration step. Although it was reduced from about 37 to 32 vol.percent, the relatively small quantity of regenerated caustic per volumeof recycled caustic employed in the extraction zone ensures.

that substantial phenolic equilibrium is maintained.

Having described the present invention, that which is sought to beprotected is set forth in the following claims.

What is claimed is:

1. In the process of removing mercaptans and thicphenols from ahydrocarbon oil additionally containing phenolic compounds, wherein saidhydrocarbons are washed with an alkaline solution, the improved methodof conserving alkali which comprises utilizing an alkaline solutioncontaining less than 1.0 wt. percent free alkali, maintaining asufiicient concentration of phenolics in said alkaline solution so as tobe in equilibrium with the phenolic compounds in said hydrocarbon oil,said alkaline solution extracting said mercaptans and thiophenols whilepreserving the amount of phenolic compounds in said hydrocarbon oil atsubstantially the same amount contained therein prior to washing withsaid alkaline solution, and directly recycling at least a portion ofsaid alkaline extracting solution for further contact with saidhydrocarbon oil.

2. The improved method of claim 1 wherein said hydrocarbon oil is acatalytically cracked naphtha, and said alkaline solution is regeneratedby an oxidation reaction.

3. The improved method of claim 1 wherein said alkaline solution iscontacted at total of 5 to 1000 volumes of oil per volume of solutionprior to being subjected to regeneration.

4. An improved process for removing mercaptans from a hydrocarbon oilwhich also contains phenolic materials which comprises: contacting saidhydrocarbon oil with an alkaline solution containing a suflicientquantity of phenolics so as to be in equilibrium with the concentrationof phenolic materials in said hydrocarbon oil, said alkaline solutionfurther containing less than 1.0 wt. percent free alkali, mercaptansthus being extracted by said alkaline solution while preserving theamount of phenolic materials in said oil at substantially the sameamount contained therein prior to contacting with said alkaline solutionrecycling at least a portion of said alkaline solution for furthercontact with said hydrocarbon oil; passing alkaline solution containingsaid extracted mercaptans to an oxidation zone where 7 said mercaptansare converted to disulfides; separating said disulfides from theeilluent of said oxidation zone, and recycling the remaining efiluentcomprising alkaline solution containing phenolics for further contactwith hydrocarbon oil.

5. The process of claim 4 wherein said hydrocarbon oil is a naphthafraction and said alkaline solution is a solution of sodium hydroxide.

6. The process of claim 4 wherein air is employed as the oxidizing agentin said oxidation zone.

7. A process for removing mercaptans and thiophenols from a crackednaphtha containing about 0.134 wt. percent phenolics, about 0.011 wt.percent thiophenols and about 0.007 wt. percent mercaptans whichcomprises contacting a volume of caustic solution containing about 31vol. percent acid oils and about 2 wt. percent free caustic with about10 volumes of said naphtha, segregating said caustic solution and saidtreated naphtha, repeating the contacting and segregating steps withsaid recovered caustic solution and additional amounts of said untreatednaphtha until said caustic solution has contacted about 200 volumes ofsaid naphtha and its free caustic content substantially reduced, andcontacting said resulting caustic solution with additional amounts ofsaid untreated naphtha whereby mercaptans and thiophenols are extractedfrom said untreated naphtha while the amount of phenolics in saidnaphtha remains substantially the same as contained therein prior tocontact with said caustic solution.

8. The process as in claim 7 wherein said volume of caustic solution isregenerated after oontacting about 250 volumes of said naphtha.

References Cited in the file of this patent UNITED STATES PATENTSYabrofi et a1 Mar. 7, 1939 Yabrolf et a1. May 28, 1940 Yabrofi et a1Dec. 3, 1940 Caselli et a1. Oct. 7, 1941 Ten Have Sept. 18, 1956 Bettset a1 Nov. 6, 1956 McNeill et a1. Aug. 5, 1958 OTHER REFERENCES 5'Caustic-Methanol Treatment of Naphtha, Library Bulletin of Abstracts;Vol. 16, No. 47, Nov. 19, 1941, pp. 187-8. Universal Oil ProductsCompany, Chicago, Ill.

Happel et al.: Critical Analysis of sweetening 10 Processes andMercaptan Removal, Petroleum'Refiner, vol. 21, No. 11, November 1942;pp. 102 (406)-109 Urban et a1 Jan. 12, 1960

1. IN THE PROCESS OF REMOVING MERCAPTANS AND THIOPHENOLS FROM AHYDROCARBON OIL ADDITIONALLY CONTAINING PHENOLIC COMPOUNDS, WHEREIN SAIDHYDROCARBONS ARE WASHED WITH AN ALKALINE SOLUTION, THE IMPROVED METHODOF CONSERVING ALKALI WHICH COMPRISES UTILIZING AN ALKALINE SOLUTIONCONTAINING LESS THAN 1.0 WT. PERCENT FREE ALKALI, MAINTAINING ASUFFICIENT CONCENTRATION OF PHENOLICS IN SAID ALKALINE SOLUTION SO AS TOBE IN EQUILIBRIUM WITH THE PHENOLIC COMPOUNDS IN SAID HYDROCARBON OIL,SAID ALKALINE SOLUTION EXTRACTING SAID MERCAPTANS AND THIOPHENOLS WHILEPERSERVING THE AMOUNT OF PHENOLIC COMPOUNDS IN SAID HYDROCARBON OIL ATSUBSTANTIALLY THE SAME AMOUNT CONTAINED THEREIN PRIOR TO WASHING WITHSAID ALKALINE SOLUTION, AND DIRECTLY RECYCLING AT LEAST A PORTION OFSAID ALKALINE EXTRACTING SOLUTION FOR FURTHER CONTACT WITH SAIDHYDROCARBON OIL.